The present invention generally relates to electrical energy management systems. More particularly, the invention relates to energy accumulation systems for use in conjunction with generative power sources.
Due to developments in power electronics and an increase in the requirements for emergency and stored energy, higher voltages and power levels are required from batteries. Various electrical functions in aerospace and ground vehicles (tactical vehicles and automotive) may be performed with power systems which incorporate energy storage elements such as batteries, supercapacitors and/or fuel cells. These applications may include aircraft APU and main engine start and ground vehicle traction drives. Energy accumulators may be employed for effective harnessing or mitigation of bidirectional flow of electric energy and improving overall efficiency.
Some large scale energy storage systems may also be employed to satisfy mobile power and distributed renewable applications for peak power shaving and/or effectively using stored energy to avoid high cost of electricity during higher rates imposed by smart metering. In many of these applications, elementary battery cells may be connected in series so that an output terminal voltage equals a sum of elementary voltages.
In addition, many newer systems may employ lithium ion (Li-ion) batteries which consist of multiple cells in series. Increased output voltage may require an increased number of series connected cells. Conventional Li-ion systems require charge/discharge control and equalization at the cell level which may require considerable electronics, particularly in the case of higher voltage batteries.
It is known that for a particular power level, the current required from a higher voltage energy storage system is lower than that of a low voltage storage system. Thus high voltage systems provide improved efficiency and overall reduced weight, volume and cost. Traditionally, high voltage energy storage systems have been realized by a) connecting a large number of energy storage cells in series; or, b) using a low-voltage energy system and connecting it to an external DC-DC converter to boost the low-voltage voltage to the desired high voltage level. In the first instance, regulation, protection and overall reliability and availability of the energy storage system becomes a major issue and it becomes imperative to assure that degradation of any one cell (e.g. excessive capacity fading, voltage imbalance, open cell) does not lead to the failure of the entire energy storage system. In the second case the low voltage battery system requires additional external circuitry for its protection. Thus, overall energy storage system weight, volume and efficiency may be negatively impacted due to wide-variation of low-voltage range and various single point failures, both in the energy storage system and cascaded DC-DC conversion.
As can be seen, there is a need for self regulated and self protected high voltage energy storage system with integrated fault-tolerant power management system to reduce overall count of number of energy storage cells and to achieve high reliability and availability of power by eliminating single point failures